1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a semiconductor device having a plurality of lead frames in which semiconductor pellets respectively mounted on the lead frames are wire-bonded to be electrically connected to each other.
2. Description of the Related Art
FIGS. 1 and 2 show a semiconductor device in which semiconductor pellets are mounted on a plurality of conventional lead frames and the semiconductor pellets are bonded to each other by wirings to be electrically connected. The same reference numerals in FIG. 1 denote the same parts as in FIG. 2.
A conventional device shown in FIG. 1 will be described below. Semiconductor pellets 2.sub.1 and 2.sub.2 are fixed on lead frames 1.sub.1 and 1.sub.2, respectively. Electrodes 4.sub.1 and 4.sub.2 made of aluminum are formed on the semiconductor pellets 2.sub.1 and 2.sub.2, respectively. The semiconductor pellets 2.sub.1 and 2.sub.2 are electrically connected to each other through a dummy lead 1a by two wires 3.sub.1 and 3.sub.2 made of gold.
The semiconductor pellets 4.sub.1 and 4.sub.2 are electrically connected to each other through the dummy lead 1a by the gold wires 3.sub.1 and 3.sub.2 due to the following reason.
That is, in a conventional device as shown in FIG. 2, when the aluminum electrodes 4.sub.1 and 4.sub.2 are directly connected by the gold wires 3, a portion of either aluminum electrode is necessarily wedge-bonded to the gold wire 3. In FIG. 2, the gold wire 3 is wedge-bonded to the aluminum electrode 4.sub.2. The wedge-bonded portion is represented by reference numeral 7b, and a ball-bonded portion is represented by reference numeral 7a. As described above, when the gold wire 3 and the aluminum electrode 4.sub.2, i.e., gold and aluminum, are wedge-bonded, the gold and aluminum are reacted with each other to form an alloy. In progress of alloying, cracks of the gold wire 3 occur and are grown, and the gold wire 3 is finally disconnected. This adversely affects reliability of the semiconductor device. These cracks occur due to the following reason. When the aluminum electrode 4.sub.2 is heated to a temperature of 200.degree. C. to 300.degree. C. and the gold wire 3 is bonded on the electrode 4.sub.2, the contact portion between the gold wire 3 and the electrode 4.sub.2 is alloyed.
The contact portion between the gold wire 3 and the electrode 4.sub.2 is to be thin. Therefore, the gold wire 3 is used for alloying, and the thickness of the contact portion is further decreased. Then, cracks occur due to a difference between expansion coefficients of the alloy portion and the gold wire 3. For this reason, in a conventional semiconductor device, as shown in FIG. 1, using the dummy lead 1a formed by plating an Ni-Fe alloy with silver, the gold wire 3 is wedge-bonded to the dummy lead 1a, thereby electrically connecting the semiconductor pellets 2.sub.1 and 2.sub.2 by wire-bonding. That is, the amount of alloying in a portion 7b where gold is wedge-bonded to silver is less than it would be between gold and aluminum. The amount of gold of the wires 3.sub.1 and 3.sub.2 for alloying gold and silver is smaller than that for alloying aluminum and gold, and cracks of the wires are less likely to occur.
Note that, since the gold wire 3 is used at the ball-bonded portion 7a in its longitudinal direction, cracks do not occur.
However, in a conventional device shown in FIG. 1, since the dummy lead 1a is required, the number of pins of the semiconductor device is increased, thereby disadvantageously increasing the size of a package. Especially, in a semiconductor device in which the number of pins and the dimensions of a package are limited, the above conventional device is difficult to use.
A means for solving the above problem has been studied. For example, as a method of using a dummy lead without an increase in number of pins and dimension of a package, it is studied that when the dummy lead is formed within the defined pitch between the original pins, it is molded, and disconnected. However, in this method, this dummy lead is necessarily, locally exposed and left outside the mold. For this reason, surface leakage is increased to pose a problem on safety standards. In order to prevent exposure of the cut portion of the dummy lead outside the mold, a double mold in which the dummy lead is molded again after cutting it is considered. However, the double mold causes a semiconductor device to be large in size. Even when the mounting density of elements is increased without increasing the dimension of the semiconductor device, freedom of frame design is degraded, thereby increasing cost.
In addition, as a method not using a dummy lead, it is studied that gold is used as a material for an electrode formed on a semiconductor pellet. However, in this method, since the gold has a large diffusion coefficient, the gold is diffused in the semiconductor pellet. Therefore, the gold and silicon contained in the semiconductor pellet are reacted with each other, thereby changing characteristics of elements inside the pellet.